DNS Overview

At one time, hostname to IP address mapping was handled by one large, centrally managed text file that contained an entry for every host accessible on the Internet. Each site downloaded a copy of the file periodically to get the latest hostname information. That scheme quickly became unwieldy, and the DNS service was conceived. It was defined in RFC 882 in 1983, and introduced two key ideas: the data is distributed and the naming of hosts is hierarchical. Making the data distributed means that every site updates its own information, and the updates become available almost immediately. Hierarchical naming prevents hostname conflicts and gives us the current domain-naming system that we are all very familiar with today. Each site obtains at least one domain name, and all of the hosts at that site are named by prefixing the simple hostname to the site's domain name. For example, a site that controls the domain name example.com might have any number of hosts with names like server1.example.com, hp4100.example.com, or www.example.com.

Each domain has at least two domain nameservers that are considered authoritative for the domain. Authoritative nameservers should have direct access to the database that contains all the information about a domain.

The data is comprised of different types of records called resource records. Different resource records provide different kinds of information, such as IP addresses, nameservers, hostname aliases, and mail routing. The resource records you need to know about for this discussion are the following:

A

The mapping of names to IP addresses is handled by A records. These records contain a hostname and its IP address. The names that people use to refer to hosts have to be converted to IP addresses used for Internet routing. A records provide this name-to-address translation.

CNAME

Some hostnames are aliases that point to other hostnames, rather than to IP addresses. This can be useful for directing requests to services (such as HTTP or POP) that might reside on systems generally known by a different name. The CNAME record provides the "real," or canonical, name that an alias hostname points to. For example, an administrator might publicize the hostname www.example.com, which is really a CNAME record pointing to server1.example.com most of the time. But during periods of maintenance on server1.example.com, for example, www.example.com could temporarily point to server2.example.com.

MX

MX records provide mail-routing information. They specify mail exchangers for domainsthat is, the names of the mail hubs that handle all the mail for a domain name. The MX records tell MTAs where to send messages. Since a domain can have multiple mail exchangers, MX records include a preference value to designate the order of priority when selecting a mail exchanger to deliver messages to.

PTR

PTR records provide a reverse lookup of IP addresses to hostnames. These records normally match up with A records, so that forward lookups of hostnames return an IP address whose reverse lookup returns the hostname. However, many hostnames can point to the same IP address, so PTR records should map back to the canonical name associated with the IP address. Some applications use PTR records as a form of authentication to make sure that a connecting client's IP address maps to the expected hostname.